For daily wit & wisdom, sign up for the Almanac newsletter.
The next Blue Moon occurs on August 30, 2023! What is a Blue Moon, exactly? Read all about the different types of Blue Moons.
What Is a Blue Moon?
Need a refresher on what a Blue Moon is? We get a lot of questions about the term, including:
Is “Blue Moon” a scientific term used in astronomy?
Did this term originate with Native American folklore, like a number of the other full Moon names?
Does a Blue Moon really look blue?
In truth, the answer to all of these questions is “no.”
The modern understanding of “Blue Moon” only took off in the 1980s. It was a result of a much earlier mistake printed in a 1946 issue of Sky & Telescope magazine, and since then, the term has gone viral in the media.
Two Types of Blue Moons
There are two definitions of the term that are commonly used today:
Seasonal Blue Moon: The extra full Moon that occurs within an astronomical season. One season—defined by the dates of the solstices and equinoxes—typically has three full moons occur within it. If a season instead has four full moons, then the third full moon (not the fourth) in the season may be called a Blue Moon.
Calendrical Blue Moon: The second full moon to occur in a calendar month. It takes our Moon about 29.5 days to complete one cycle of phases (from new Moon to new Moon), so if a full Moon occurs on the first of a month, there will be a second full Moon—a Blue Moon—at the end of the month, too (except in February).
Although the latter definition is the one more commonly followed today, the former actually came first. As mentioned above, a misinterpretation of the seasonal definition in the 1940s gave way to the calendrical definition, which was later popularized in the 80s and has stuck around to today.
The Blue Moon in August 2023 will be a calendrical Blue Moon (i.e., the second full Moon in a calendar month).
After that, the next Blue Moon we’ll experience is a seasonal Blue Moon (the third of four full Moons in one astronomical season) on August 24, 2024.
How Often Does a Blue Moon Occur?
Most months have one full Moon, not two.
Since the Moon’s period of phases is 29 1/2 days, while months usually have 30 or 31 days, it’s obvious that if a full Moon lands on the first day of any month except February, it will repeat again at the end.
Turns out, calendrical Blue Moons happen every 30 months on average. Two and a half years. Seasonal Blue Moons happen at a similar rate: about once every two to three years. So maybe “once in a Blue Moon” isn’t so rare after all!
Two Blue Moons in a Single Year
How often do we have two Blue Moons in a single year? (As in 2018, when there were Blue Moons in both January and March, and no full Moon in February.)
This won’t happen again until 2037, when we’ll again have another Blue Moon in both January and March.